1.0.0-beta4

Added

• Spi\Volatile, a marker for stores with nothing to roll back; the
  in-memory stores carry it and the identity-unit-of-work warning is decided
  by it rather than by class name. ServerBuilder::check(Services) returns
  the same findings for a host's status page.
• Testing\RacingActionRequestStore: stages a decision lost to another
  worker, outright or through a host callback that flips its own row first.
• Testing\FixedClock::set() for jumping to an absolute instant.
• Client\DeliveryVerdict: the retry classification every outbox worker
  needs (Retry for transport failures, 5xx, 408 and 429; Reject
  otherwise), and a guide section specifying the outbox row, claim and
  outcome model two hosts converged on.

Changed

• ActionRequestStatusChanged and the status notification fire after the
  decision's side effects (grants, revision, revocation) are in place, once
  per decision, with the status the request had before it; a change that
  fails to apply reports Failed from Pending even though the store
  recorded Accepted in between (Drupal integration review, round 2).